Chip-scale star tracker

ABSTRACT

A chip scale star tracker that captures plane-wave starlight propagating in free space with a wafer-thin angle-sensitive broadband filter-aperture, and directs the light into a waveguide structure for readout. Angular information about the star source is determined from characteristics of the starlight propagating in the waveguide. Certain examples include internal propagation-constant-based baffling to elimination stray light from extreme angles.

BACKGROUND

Traditional star trackers are high-performance imaging instruments thatcontain the optical and processing elements typically found in camerasystems, such as an imaging sensor focal plane and a telescope. Forfar-field imaging, the diffraction equation establishes a directrelationship between maximum object angular resolution (and thus imagingperformance) and the diameter of the optical aperture. Accordingly, thetelescope of a star tracker must generally have a fairly large aperturediameter in order to collect a sufficient amount of starlight to achievereasonable imaging performance. As a result, the focal length (andphysical length) of the telescope must be substantial as well. Thus,traditional star trackers are typically large in size, and bulky.

SUMMARY OF INVENTION

Aspects and embodiments are directed to a miniaturized star tracker(also referred to as a star camera) in which traditional optics arereplaced with a wafer-based structures, providing a system that has alarge light collection aperture on a wafer-thin substrate. Thiscombination may enable conformal mounting on a spacecraft or othervehicle. According to certain embodiments, and as discussed in moredetail below, the “chip-scale star tracker” captures plane-wavestarlight propagating in free space with a wafer-thin angle-sensitivebroadband filter-aperture, and directs the light into a lightguidestructure for readout. In some example, the chip-scale star tracker isless than 1 millimeter thick, thus providing a true wafer processsolution to an optical imaging task.

According to one embodiment, a star tracker comprises a lightguide, acoupling system disposed on a surface of the lightguide and configuredto couple starlight into the lightguide such that a mode of propagationof the starlight in the lightguide is at least partially determined byan angle of incidence of the starlight on the coupling system, and adetector system coupled to the lightguide and configured to analyze thestarlight propagated in the lightguide to determine at least one of anazimuth angle and an elevation angle of a star that is a source of thestarlight.

In one example of the star tracker, the lightguide is a multimodewaveguide comprised of at least one dielectric layer of a wafersubstrate. The wafer substrate may be a silicon wafer, for example. Arefractive index of the at least one dielectric layer may be tapered toprevent propagation in the lightguide of light incident on the couplingsystem from a predetermined range of angles relative to the normal tothe surface of the lightguide. In one example the coupling systemcomprises a broadband grating coupler patterned on the surface of thelightguide. In another example the wafer substrate includes a pluralityof output optical apertures connected to the lightguide, and the startracker further comprises an interferometer selectively coupled to apair of the plurality of output optical apertures and configured todetermine a phase difference between the starlight propagated via thelightguide to each of the pair of the plurality of output opticalapertures. The star tracker may further comprise an optical switchcoupled to the plurality of output optical apertures and configured toselectively couple the pair of the plurality of output optical aperturesto the interferometer. The star tracker may further comprise a processorcoupled to the interferometer and configured to reconstruct an imagefrom phase information determined by the interferometer. In one examplethe plurality of output optical apertures are arranged along two axesthat intersect one another at an angle of approximately 90 degrees.

According to another embodiment a method of imaging a star fieldcomprises coupling starlight from at least one star into a planarlightguide, propagating the starlight via the lightguide to a detectorsystem, wherein a mode of propagation of the starlight in the lightguideis based at least in part on an angle of incidence of the starlight on asurface of the lightguide, and determining at least one of an azimuthangle and an elevation angle of the at least one star based on detectedcharacteristics of the starlight.

In one example of the method coupling the starlight into the lightguideincludes coupling the starlight into the lightguide using a gratingcoupler patterned on the surface of the lightguide. In another examplepropagating the starlight includes propagating the starlight via thelightguide to a pair of optical apertures. In this example the methodmay further comprise interferometrically measuring a phase differencebetween the starlight at each of the pair of optical apertures.

According to another embodiment a star tracker comprises a wafersubstrate including at least one dielectric layer, a coupling structurepatterned on a surface of the wafer substrate and configured to couplestarlight into the at least one dielectric layer of the wafer substrate,at least one readout waveguide configured to propagate the starlightcoupled into the at least one dielectric layer by the couplingstructure, wherein a mode of propagation of the starlight in thewaveguide is at least partially determined by an angle of incidence ofthe starlight on the coupling structure, and a detector system coupledto the at least one readout waveguide and configured to analyze thestarlight propagated in the multimode waveguide to determine at leastone of an azimuth angle and an elevation angle of a star that is asource of the starlight.

In one example the coupling structure is circularly symmetric on thesurface of the wafer substrate. In another example the at least onereadout waveguide includes a plurality of readout waveguides positionedaround the coupling structure along a perimeter of the wafer substrate.The wafer substrate may be a silicon wafer, for example. The couplingstructure may include a broadband waveguide grating coupler, forexample. In one example the star tracker further comprises a pluralityof optical apertures positioned on the wafer substrate and coupled tothe at least one readout waveguide, an interferometer coupled to thedetector system, and an optical switch coupled to the plurality ofoptical apertures and configured to selectively couple a pair of theplurality of optical apertures to the interferometer. The star trackermay further comprise a corresponding plurality of optical path lengthcontrol elements, each associated with a respective one of the opticalapertures.

Still other aspects, embodiments, and advantages of these exemplaryaspects and embodiments are discussed in detail below. Embodimentsdisclosed herein may be combined with other embodiments in any mannerconsistent with at least one of the principles disclosed herein, andreferences to “an embodiment,” “some embodiments,” “an alternateembodiment,” “various embodiments,” “one embodiment” or the like are notnecessarily mutually exclusive and are intended to indicate that aparticular feature, structure, or characteristic described may beincluded in at least one embodiment. The appearances of such termsherein are not necessarily all referring to the same embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of at least one embodiment are discussed below withreference to the accompanying figures, which are not intended to bedrawn to scale. The figures are included to provide illustration and afurther understanding of the various aspects and embodiments, and areincorporated in and constitute a part of this specification, but are notintended as a definition of the limits of the invention. In the figures,each identical or nearly identical component that is illustrated invarious figures is represented by a like numeral. For purposes ofclarity, not every component may be labeled in every figure. In thefigures:

FIG. 1 is a schematic diagram of one example of a chip-scale startracker according to aspects of the invention;

FIG. 2A is a schematic diagram showing a perspective view of waveguidegrating coupling;

FIG. 2B is a schematic diagram showing a top view of the waveguidegrating coupling, corresponding to FIG. 2A;

FIG. 3 is a schematic diagram of another example of a chip-scale startracker according to aspects of the invention;

FIG. 4A is a diagram of one example of a notional supermodal exitpattern corresponding to light coupled into the waveguide from a firstinput angle;

FIG. 4B is a diagram of another example of a notional super modal exitpattern corresponding to light coupled into the waveguide from a second,different, input angle;

FIG. 5A is a schematic diagram showing a top view of another example ofa chip-scale star tracker according to aspects of the invention;

FIG. 5B is a schematic diagram showing a top view of another example ofa chip-scale star tracker including light baffles according to aspectsof the invention;

FIG. 6 is a schematic diagram of another example of a chip-scale startracker according to aspects of the invention;

FIG. 7 is a schematic diagram of another example of a chip-scale startracker according to aspects of the invention; and

FIG. 8 is a schematic diagram of another example of a chip-scale startracker incorporating white light interferometry according to aspects ofthe invention.

DETAILED DESCRIPTION

As discussed above, traditional star trackers are typically large due tothe need for a large optical aperture to achieve sufficiently highresolution imaging, which generally results in the system having a largefocal (and physical) length. However, in many applications it may bedesirable to minimize the size and weight of the star tracker system. Ina chip-scale star tracker system according to aspects and embodiments ofthe present invention, focal length has no meaning because the light isnot imaged as in a traditional lens or mirror based system, but isinstead coupled and filtered into a planar light-guide structurepatterned on a layered wafer substrate. The captured light propagateswithin the wafer material and is detected at the edges of the wafer withlow noise photo-detectors. The detected light is analyzed to obtain thedetailed propagation characteristics which determine the star angle, asdiscussed further below. Thus, aspects and embodiments are directed to astar tracker in which waveguide-based light collection and analysistechniques are used to remove the need for a large telescope, therebyachieving a system that retains a large optical aperture for lightcollection, while eliminating many of the large and sometimes heavyoptical elements associated with traditional star trackers.

It is well established that free-space light can be coupled into awaveguide through the use of a diffraction grating disposed on a surfaceof the waveguide. This concept is known as “grating coupling.” Aspectsand embodiments are directed to a chip-scale star tracker that uses theconcept of grating coupling to collect starlight with a planarwaveguide, thereby removing the need for a traditional optical telescopeto collect and focus the light. Unlike traditional star trackers,embodiments of the chip-scale star tracker discussed herein do notdirectly “image” the stars, but rather couple the starlight into lightguides which are then interrogated for star angle information, asdiscussed further below. Particular coupling modalities may be used toextract information from the collected starlight and create images ofindividual stars or a star field. Additionally, “baffling,” or theelimination of stray light from extreme angles which would otherwisecontribute to noise in the measurements, may be accomplished inside thelight guides. Furthermore, according to certain embodiments, theincorporation of white-light interferometry into a chip-scale starcamera platform may provide improved angular accuracy to star imaging,as also discussed in more detail below. These and other aspects mayprovide for a chip-scale star tracker in which the complete opticalsystem may be as thin as a silicon wafer.

It is to be appreciated that embodiments of the methods and apparatusesdiscussed herein are not limited in application to the details ofconstruction and the arrangement of components set forth in thefollowing description or illustrated in the accompanying drawings. Themethods and apparatuses are capable of implementation in otherembodiments and of being practiced or of being carried out in variousways. Examples of specific implementations are provided herein forillustrative purposes only and are not intended to be limiting. Also,the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use herein of“including,” “comprising,” “having,” “containing,” “involving,” andvariations thereof is meant to encompass the items listed thereafter andequivalents thereof as well as additional items. References to “or” maybe construed as inclusive so that any terms described using “or” mayindicate any of a single, more than one, and all of the described terms.Any references to front and back, left and right, top and bottom, upperand lower, and vertical and horizontal are intended for convenience ofdescription, not to limit the present systems and methods or theircomponents to any one positional or spatial orientation.

Referring to FIG. 1, there is illustrated a schematic diagram of oneexample of the optical system for a chip-scale star tracker according toone embodiment. The optical system 100 of the chip-scale star trackerincludes a multi-layer wafer substrate 110, at least one layer of whichacts as a light guide (or waveguide) acts as a planar dielectricwaveguide 115 to propagate starlight 130, 135 in one or more spatialpropagation modes, as discussed further below. In one embodiment, thewaveguide 115 is a multi-mode waveguide capable of supporting multiplespatial modes of propagation of light. In one example, the wafersubstrate 110 is a sub-millimeter-thin wafer. The wafer materials mayinclude any material that has broadband optical transparency such thatit is capable of transmitting the starlight 130, 135. In certainexamples the waveguide includes one or more dielectric layers of thewafer substrate 110; however, in other examples the waveguide may be ahollow waveguide, or may include metallic grating structures,metamaterial structures, or other waveguide forms.

The wafer substrate 110 includes a coupling structure 120, which mayinclude an array of micro-optical elements, configured to couple thestarlight 130, 135 into the waveguide 115. In one example, the couplingstructure 120 is patterned on the surface of the wafer substrate 110. Inone embodiment, the coupling structure 120 includes a sub-wavelengthbroadband grating configured to couple the broadband starlight 130, 135into the waveguide 115. However, it will be appreciated by those skilledin the art, given the benefit of this disclosure, that embodiments ofthe chip-scale star tracker are not limited to the use of a gratingcoupler. For example, the coupling structure 120 may include any ofwaveguide gratings, optical nanoantennas, photonic crystals,nanomaterials, negative-refractive index materials, and the like.

Starlight 130, 135 emanates from stars 140 as an initially approximatelycircular wavefront. However, since the stars 140 are very distant, andbecause the optical system 100 is small, the starlight 130, 135 isessentially a perfect plane wave when it arrives at the optical system.Referring to FIGS. 2A and 2B, and considering the starlight 135 as anexample, the free space plane wave 135 a is perturbed by abroadband-coupler array structure 120 patterned on the surface of thewafer substrate 110, as discussed above, thereby redirecting the coupledlight 135 b into layers of light guides located underneath the couplingstructure 120 and that are oriented roughly perpendicular to the wafernormal. The redirected light 135 b is distributed into combinations ofpropagation paths based on the original approach angle θ and thewavelength of the light. For example, referring to FIG. 3, whichillustrates an example of an optical system wherein the wafer substrate110 includes multiple layers beneath the coupling structure 120,starlight 130, 135 incident on the coupling structure 120 from differentinput angles may be coupled into different spatial modes in the wafer110. The redirected light 130 b, 135 b travels in a “supermodal pattern”along the light guides to the edge of the wafer substrate 110, asillustrated schematically in FIGS. 2B and 3. The supermodal pattern maythen be sampled by photo-sensitive detectors (not shown), to captureprecise modal characteristics from which propagation constants and thenapproach angles (θ) can be derived. FIGS. 4A and 4B illustrate notionalsupermodal exit patterns at the output of the light guide(s)representing two different input angles of the starlight. In oneexample, the photo-sensitive detectors are very-low-noise sensor pixels,optionally positioned both at the edge and under the wafer. Thus, thediscrete modes of the waveguide provide angular imaging resolution, andstar azimuth and elevation angle may be determined accurately from theangular extent of light propagating in the wafer substrate 110.

An important design feature in all star trackers or cameras is the “sunshade” and/or “baffles,” which are used to eliminate stray lightarriving at extreme angles from outside of the field of view of thecamera. For the wafer-thin chip-scale star tracker embodiments discussedherein, an external sun shade, as may be used with conventional startrackers, may be far too large. However, direct sunlight impinging onthe optics plane may result in far too high a level of background noisethat would negatively impact the imaging performance of the startracker. Accordingly, in certain embodiments of the chip-scale startracker, baffling is accomplished inside the light-guide structure 115of the wafer 110, with micro-optics and by modifying the effective indexof the wafer through tapering. Specifically, intra-chip baffles are usedto maneuver stray light out of the detection path. According to oneembodiment, the baffles operate by segregating light based on thepropagation constant, as discussed further below. Thus, light outside ofthe numerical aperture of the layer structure will not be coupled intothe detection layers, while vertical propagation path shifts remove anyscattered light from the detection plane.

Referring to FIGS. 5A and 5B, there are illustrated, schematically, topviews of two examples of a chip scale star tracker according to certainembodiments. FIG. 5A illustrates an example of the basic chip-scale startracker concept as discussed above with respect to FIGS. 1 and 3, andFIG. 5B illustrates an example of a chip-scale star tracker includingtapered-index internal baffles. Referring to FIG. 5A, starlight 130arriving at an angle (illustrated as a near-normal angle of incidence)with the field-of-view of the system is coupled into the planarwaveguide 115 by a grating coupler (or other coupling device) 120 andpropagates in the waveguide, as discussed above. However, stray light,such as sunlight 315 from the sun 310, is also coupled into thewaveguide 115 via the coupling structure 120 and may propagate to theoutput of the waveguide along with the starlight 130, as shown.

To address this concern, the waveguide may be tapered and designed tostrongly favor light in the field-of-view, while discarding most straylight arriving from outside of the field-of-view, as shown in FIG. 5B.Specifically, according to one embodiment, the waveguide 320 is formedwith a tapered refractive index such that certain spatial modes do notpropagate to the output of the waveguide. Those skilled in the art willappreciate, given the benefit of this disclosure, that the waveguide 320may not have a physical taper, as illustrated, but rather the “taper” isachieved by varying the refractive index of the layer(s) used to formthe waveguide in the wafer 110. As discussed above, at least for certainangles of incidence of the starlight 130, 135, the coupled lightpropagates through the waveguide in a “zig-zag” manner, as illustratedin FIG. 2B, with the light waves confined to the dielectric by totalinternal reflection at its surface. Light coupled into the waveguide 320from different angles will propagate in different spatial modes, eachmode having a different propagation constant. The propagation constantdefines the angle or “sharpness” of the zig-zag path taken by the lightwaves as they propagate through the waveguide 320. The refractive indexof the material(s) forming the waveguide 320 may be selected such thatlight coupled in from very high angles of incidence (relative to thenormal to the coupler), and thus having a propagation constant thatdefines a very “sharp” zig-zag (one having a high rate of repetition),fails the condition of total internal reflection necessary to supportpropagation through the waveguide. Accordingly, such light 315 isrejected out of the waveguide 320, as shown in FIG. 5B. In practicalimplementations of the design concept illustrated in FIG. 5B, thetapered structure may have stray light suppression characteristics inboth azimuth and elevation.

Advances in wafer-fabrication capabilities allow for patterning thesub-wavelength array coupling structures 120 (in certain embodiments.effectively complex gratings) that are needed for angle-sensitivecoupling to freely propagating broad-band light, as discussed above. Byfurther angular filtering of this light, for example, by scanning thearray coupling coefficient, and filtering in a planar layer stack, theinput angle of the coupled plane wave may be detected with extremeangular resolution. In particular, according to one embodiment, thedirectionality of the grating coupling may be manipulated by realtimetuning (for example, by thermal tuning or charge-injection tuning,etc.), so that the coupling envelope scans across the star point source140, providing an enhancement in angular resolution over the pointsource. The incoming plane wave is then distributed into lightguidelayers with a propagation constant distribution, and the supermodalpattern is spatially sampled at the sensor planes, providing goodangular readout sensitivity as discussed above. Additionally, in certainembodiments, particular tuning and scanning techniques (such as, but notlimited to, charge-injection tuning, etc.) may permit coherencemanipulation for improved performance. According to certain embodiments,the above discussion provides “elevation” angle resolution. In oneembodiment, azimuthal selectivity is achieved through acircular-symmetry design approach, as discussed below.

Referring to FIG. 6, there is illustrated (schematically) an example ofa waveguide-based star tracker system that employs circular symmetry toenable both elevation and azimuth angular resolution. In this example,the system 400 includes a wafer substrate 410 that is patterned with acoupling system 420 that is surrounded at the perimeter of the substrate410 by a plurality readout waveguides 430. In the illustrated example,the coupling system 320 is a circular waveguide grating, and the readoutwaveguides 430 are linear arrays. In one example, the substrate 410 is asilicon wafer; however, other materials may be used as discussed above.At least one of the readout waveguides 430 is coupled to readoutcircuitry 440, including a photodetector and associated circuitrycapable of analyzing the light 130 propagated by the readout waveguideto determine the azimuth and elevation of the star 140 as discussedabove. Each of readout waveguides 430 may be positioned and configuredto accept a particular mode of propagation, or coupled light incident onthe coupling system 420 from a particular angle in elevation or azimuth.Thus, the circular coupling system 420 may be configured tonear-hemispherical light collection, and the array of readout waveguides430 may be used to distribute the coupled light into azimuth andelevation “bins.” The light propagated by each readout waveguide 430 maybe detected and the resulting signal processed by the associated readoutcircuitry 440. The signals from each readout circuitry 440 may beprocessed to produce an image of the star field, for example, from theazimuth and elevation information extracted from the light propagated bythe readout waveguides 430.

The coupling system 420 may include tapering, as discussed above withreference to FIG. 5B, to reject stray light arriving from outside of thefield-of-view of the readout circuitry 440 associated with acorresponding readout waveguide 430. For example, starlight 130 whichmay be approximately normally incident (in elevation angle) on thecoupling system 420 may propagate via a readout waveguide 430 to thereadout circuitry 440. However, sunlight 315 which may incident at ahigh angle (in elevation) relative to the normal to the surface of thecoupling system 420 may be rejected by the taper. According to certainexamples, the coupling system 420 may be tuned to favor light fromcertain elevation angles, for example, by tuning the refractive index ofthe waveguide material to change the taper characteristics based onthermal or electrical tuning. Thus, the coupling system 420 may beconfigured to accept starlight 130 from certain elevation angles, whilerejecting sunlight 315 from other elevation angles, even the azimuthangle is the same or very similar.

In the example illustrated in FIG. 6, the coupling system 420 iscircularly symmetric to allow for near-hemispherical light collection.In another embodiment, the coupling system may be configured and tunablefor directional scanning. For example, referring to FIG. 7 there isillustrated another example of a star tracker system in which the wafer410 includes tunable coupling structures 450 for directional scanning.Although not illustrated in FIG. 7, the coupling structures 450 may alsobe tapered as discussed above to favor light from certain input angles.

Thus, aspects and embodiments may provide a wafer-thin star tracker thatremoves the need for the large telescope optics and baffles typicallyused in conventional star camera systems in favor of a light-guidesystem in which free-space light is coupled into a wafer or waveguide.In particular, aspects and embodiments provide a star tracker in whichstarlight is coupled into a light guide that confines the starlight andguides it to detectors that map the angular extent of the confinedpropagation. From this angular map, the azimuth and elevation angles ofthe target stars may be determined. No image-forming/focusing optics, orsensor arrays, may be used in the system; instead, star elevation andazimuth information is gleaned from the characteristics of lightpropagating in the guide(s). As discussed above, the coupling system mayinclude any of photonic crystals, nanomaterials, nanoantennas,negative-refractive index materials, optical antennas, and waveguidegratings.

In addition, certain aspects and embodiments are directed to awafer-scale long-baseline broadband multi-aperture interferometricimaging system, and in particular, to the use of on-chip white-lightinterferometry in a chip-scale star tracker. In one embodiment, such animaging system includes a coupling system that may be used to couplestarlight into a wafer as discussed above. The wafer includes multipleoutput apertures positioned throughout the wafer that are switched intovarious baseline combinations, and which lead into chip-scale whitelight interferometers. Each interferometer may be configured todetermine the phase difference between two apertures at a time (a singlepoint in the Fourier domain). An image may then be reconstructed frommany different baseline pairs of apertures. In one embodiment, on-chipphase tuning may be implemented for path length matching, as well asspectral sampling for processing the light in narrow spectral bands.Such a wafer-scale interferometric imaging capability may provide veryhigh angular resolution in imaging of the star(s) 140.

For example, referring to FIG. 8, there is illustrated (schematically) achip-scale star tracker 500 configured for white light interferometryaccording to one embodiment. The wafer substrate 510 includes aplurality of optical coupling apertures 520 that are connected to anoptical switch 530. In one example the optical coupling apertures 520are arranged along axes 540, 545 that intersect one another at an angleof approximately 90 degrees, as shown in FIG. 8. Waveguide-based phasetuning via optical path length control elements 550 may provide preciseoptical path-length matching and dispersion management. Each opticalcoupling aperture may be optically paired with any other aperture, andfed into an interferometer 560. The optical switch 530 selects whichpair of apertures is fed to the interferometer 560 at any given time. Inone example, the interferometer characterizes broadly distributedspectral fringes. Corresponding images may be reconstructed using aprocessor 570. Embodiments of the chip scale star tracker 500 may beused to cover a full range of spatial frequencies, where “full range ofspatial frequencies” is defined as the range between and including themaximum and minimum possible distance between coupling apertures 520that can be supported by the wafer geometry. Additionally, all possibleangular axes may be covered by the ˜90 degree arrangement illustrated inFIG. 8.

Thus, aspects and embodiments may provide a solution for the creation ofa highly angle-sensitive optic for reading out the arrival angle of anincoming optical plane wave, while simultaneously packaging the entireoptical path into a wafer-thin structure. As discussed above, thewafer-thin optical system interacts with the wave front, and coupleslight over a broad bandwidth to allow for star imaging. Additionally,the optical system may perform filtering and phase tuning, and may allowfor propagation-constant based baffling as discussed above. The couplingsystem may include sub-wavelength structures patterned on the waferwhich includes the light guides, as also discussed above. Thispatterning may be accomplished using modern wafer-scale optical designand fabrication processes, including silicon photonics and metallicoptical nano antennas.

Having described above several aspects of at least one embodiment, it isto be appreciated various alterations, modifications, and improvementswill readily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be part of thisdisclosure and are intended to be within the scope of the invention.Accordingly, the foregoing description and drawings are by way ofexample only, and the scope of the invention should be determined fromproper construction of the appended claims, and their equivalents.

What is claimed is:
 1. A star tracker comprising: a lightguide; acoupling system disposed on a surface of the lightguide and configuredto couple starlight into the lightguide such that a mode of propagationof the starlight in the lightguide is at least partially determined byan angle of incidence of the starlight on the coupling system; and adetector system coupled to the lightguide and configured to analyze thestarlight propagated in the lightguide to determine at least one of anazimuth angle and an elevation angle of a star that is a source of thestarlight.
 2. The star tracker of claim 1, wherein the coupling systemcomprises a broadband grating coupler patterned on the surface of thelightguide.
 3. The star tracker of claim 1, wherein the lightguide is amulti-mode waveguide comprised of at least one dielectric layer of awafer substrate.
 4. The star tracker of claim 3, wherein the wafersubstrate is a silicon wafer.
 5. The star tracker of claim 3, wherein arefractive index of the at least one dielectric layer is tapered toprevent propagation in the lightguide of light incident on the couplingsystem from a predetermined range of angles relative to the normal tothe surface of the lightguide.
 6. The star tracker of claim 1, whereinthe wafer substrate includes a plurality of output optical aperturesconnected to the lightguide; and further comprising: an interferometerselectively coupled to a pair of the plurality of output opticalapertures and configured to determine a phase difference between thestarlight propagated via the lightguide to each of the pair of theplurality of output optical apertures.
 7. The star tracker of claim 6,further comprising an optical switch coupled to the plurality of outputoptical apertures and configured to selectively couple the pair of theplurality of output optical apertures to the interferometer.
 8. The startracker of claim 6, further comprising a processor coupled to theinterferometer and configured to reconstruct an image from phaseinformation determined by the interferometer.
 9. The star tracker ofclaim 6, wherein the plurality of output optical apertures are arrangedalong two axes that intersect one another at an angle of approximately90 degrees.
 10. A method of imaging a star field comprising: couplingstarlight from at least one star into a planar lightguide; propagatingthe starlight via the lightguide to a detector system, wherein a mode ofpropagation of the starlight in the lightguide is based at least in parton an angle of incidence of the starlight on a surface of thelightguide; and determining at least one of an azimuth angle and anelevation angle of the at least one star based on detectedcharacteristics of the starlight.
 11. The method of claim 10, whereincoupling the starlight into the lightguide includes coupling thestarlight into the lightguide using a grating coupler patterned on thesurface of the lightguide.
 12. The method of claim 10, whereinpropagating the starlight includes propagating the starlight via thelightguide to a pair of optical apertures, and further comprising:interferometrically measuring a phase difference between the starlightat each of the pair of optical apertures.
 13. A star tracker comprising:a wafer substrate including at least one dielectric layer; a couplingstructure patterned on a surface of the wafer substrate and configuredto couple starlight into the at least one dielectric layer of the wafersubstrate; at least one readout waveguide configured to propagate thestarlight coupled into the at least one dielectric layer by the couplingstructure, wherein a mode of propagation of the starlight in thewaveguide is at least partially determined by an angle of incidence ofthe starlight on the coupling structure; and a detector system coupledto the at least one readout waveguide and configured to analyze thestarlight propagated in the multimode waveguide to determine at leastone of an azimuth angle and an elevation angle of a star that is asource of the starlight.
 14. The star tracker of claim 13, wherein thecoupling structure is circularly symmetric on the surface of the wafersubstrate.
 15. The star tracker of claim 14, wherein the at least onereadout waveguide includes a plurality of readout waveguides positionedaround the coupling structure along a perimeter of the wafer substrate.16. The star tracker of claim 13, wherein the wafer substrate is asilicon wafer.
 17. The star tracker of claim 13, wherein the couplingstructure includes a broadband waveguide grating coupler.
 18. The startracker of claim 13, further comprising: a plurality of opticalapertures positioned on the wafer substrate and coupled to the at leastone readout waveguide; an interferometer coupled to the detector system;and an optical switch coupled to the plurality of optical apertures andconfigured to selectively couple a pair of the plurality of opticalapertures to the interferometer.
 19. The star tracker of claim 18,further comprising a corresponding plurality of optical path lengthcontrol elements, each associated with a respective one of the opticalapertures.